1. Field of the Invention
The present invention provides a method for the coating of glass microspheres with an aluminium hydroxide.
2. Background Art
Applications for aluminium hydroxide coated glass microspheres are for example in the production of light weight glass-reinforced plastic composite materials for use in mass transportation.
Aluminium hydroxide is well-established in the plastics industry as a halogen-free filler which is used to impart fire retardant and smoke suppressing properties to synthetic thermosetting resin systems. Moreover, the trend towards stricter requirements on the burning behavior of plastics means an increasing demand for aluminium hydroxides which can be incorporated in plastics at ever higher filling levels. In glass-reinforced unsaturated polyester composites for example it is not uncommon for the aluminium hydroxide content of the composite material to be &gt;50 wt. %.
In mass transportation for instance, it is possible to achieve the strict fire requirements on composite materials by using aluminium hydroxide alone as fire retardant filler. At the same time, however, the long term trend in composite materials is towards lighter weight composites which reduce energy costs. In the railways, for example, less weight means an increased pay-load for high speed trains and less wear and tear on the tracks.
Aluminium hydroxide (Al(OH).sub.3) has a density of 2.42 g/cm.sup.3 which is considerably higher than the ca 1.1 g/cm.sup.3 of a synthetic resin. The use of aluminium hydroxide by itself therefore raises the density and hence overall weight of the end-composite. In addition, the glass fibres used for structural composites contribute further to increased weight due to the even higher density of ca 2.6 g/cm.sup.3.
Well-established in the art is the incorporation of hollow microspheres of aluminium silicate or borosilicate glass to decrease the overall weight of plastic composite materials and enhance their thermal insulation characteristics. Such microspheres are available in particle sizes which are similar to those of aluminium hydroxide but they have densities in the range of 0.1-1.0 g/cm.sup.3, so that their incorporation in even relatively small amounts can provide a significant reduction in weight of the end composite. The problem when using hollow glass microspheres however is the need to maintain acceptable mechanical properties of the end composite. This is particularly important for structural plastic composite materials. However, the well-rounded, smooth surfaces of the hollow glass microspheres militate against an effective "bond" between the inorganic filler and the organic resin so that the use of hollow of glass microspheres is generally accompanied by a deterioration in physical properties of the composite material.